oldbluetruck,
hillcitybill sure did his homework...everything he mentions is 100% true and speaks from experience.
The Rustoleum products shouldn't be confused with automotive products. If you have a oil well in your back yard, or a rusty tractor, you would be all set using the Rustoleum paints.
Anywhere you intend on putting automotive paint would need a foundation of primers and sealers that are compatible with that paint...you won't find these at ACE hardware.
Etch primer is applied to clean, bare steel. The acid component of it 'etches' the bare steel to promote adhesion and inhibit rust.
A truck frame might be a good candidate for Rustoleum, but not a body panel. Before you pick up the spray gun, do some research, read the labels on the can, and talk to the counter guy/gal at the automotive paint store.
Stuart